1. Field of the Invention
This invention relates to the cutting structure formed on rotary cones of rotary cone rock bits utilized to drill boreholes in an earthen formation.
More particularly, this invention relates to the use of shaped diamond or other ultra hard material insert cutters in the heel row of each of the rotary cones associated with the drill bit for maintaining the gage bore diameter of the formation. These ultra hard materials include cubic boron nitride and/or diamond/refractory metal carbide composites.
2. Description of the Prior Art
Diamond inserts in roller cone rock bits have been tried before in an attempt to extend the useful life of a rock bit as it works in a borehole.
U.S. Pat. No. 4,940,099 teaches the utilization of alternating tungsten carbide inserts and diamond inserts in each row formed on a rock bit cutter cone. Both the heel row and the gage row as well as successive concentric rows terminating at the apex of the truncated cone alternate tungsten carbide chisel inserts with diamond inserts. The heel row adjacent the cone mouth opening alternates flush mounted tungsten carbide inserts with harder tungsten carbide flush inserts with a layer of diamond bonded thereto. The alternating gage row inserts extend from the cone surface and serve to cut the gage of the borehole which of course determines the diameter of the drilled hole in the earthen formation.
It is well known in the art to utilize flush type inserts in the heel row of roller cones primarily to minimize erosion of the cones due to the passage of drilling fluid and formation detritus between the heel and gage rows of the cones and the borehole wall. The '099 patent, while it teaches alternating hard and soft flush inserts in the heel row also teaches that it is more important that the larger diameter rows, particularly the gage row, be provided with an intermingled pattern of soft and hard inserts to facilitate differing earthen formations.
Maintenance of a constant diameter borehole throughout the drilling operation is of paramount importance in controlling cost-per-foot drilling costs. If a rock bit should drill undergage it results in a following, same diameter bit to pinch due to the undersized hole condition. This usually results in a ruined rock bit and is the cause of another trip out of the hole followed by a reaming operation all of which is time consuming and very costly.
Flush type heel row inserts ultimately act as a passive bearing surface when the heel of the cone is in contact with the borehole wall. When the entire heel surface of each of the cones is in contact with the borehole wall, the cones are subjected to tremendous inthrust loads. The inthrust loads tend to pinch the bit, damage the cone and journal bearings and cause heat checking of the tungsten carbide inserts.
U.S. Pat. No. 5,131,480, assigned to the same assignee as the present invention and incorporated herein by reference, teaches the use of extended tungsten carbide inserts in a recessed heel row in a milled tooth rotary cone rock bit. While this patented feature greatly improved directional drilling capabilities, the rounded projections on the heel row inserts somewhat limited the rock shearing function necessary for aggressive side cutting while turning from a straight drill run.
It was found through experimentation that if drilling energy is not put into shearing the rock, the energy then converts into pushing the cone away from the rock formation resulting in the heretofore mentioned inthrust condition with all of its disadvantages.